1. Field of the Invention
This invention relates to a process for producing sorbic acid or its salt which is useful as, for example, a food additive. Particularly, the invention relates to a process for producing sorbic acid or its salt that has a minimized coloring and a minimized deterioration of hue over time.
2. Description of the Related Art
In various known processes for producing sorbic acid or its salt, a commercially important pathway is a process of polymerizing crotonaldehyde and ketene to form an intermediate polyester, and decomposing the polyester to yield sorbic acid. The polyester is decomposed, for example, by the aid of hydrochloric acid, an alkali or heat to yield a crude sorbic acid. The thus-prepared crude sorbic acid generally contains a variety of colored substances, tar substances, and other impurities with varying degrees of concentration, and is subjected to a purification operation.
As purification processes of sorbic acid or its salt, a treatment with activated carbon, recrystallization with water or with a mixture of water and an organic solvent, and distillation purification of a solution of sorbic acid or its salt in petroleum have been known. For example, Japanese Unexamined Patent Application Publication No. 54-163516 discloses a purification process. This process includes the steps of preparing a polyester from ketene and crotonaldehyde, decomposing the polyester with hydrochloric acid in the presence of, for example, a urea compound to yield a decomposition reaction mixture, separating the decomposition reaction mixture by filtration, and washing the residue to yield a crude sorbic acid, adding a sodium hydroxide aqueous solution to the crude sorbic acid to yield a sodium sorbate aqueous solution, treating the aqueous solution with activated carbon, neutralizing the treated solution, and cooling the neutralized solution to crystallize sorbic acid.
Japanese Examined Patent Application Publication No. 44-26646 discloses a process for producing a crystalline sorbic acid. The process includes the steps of preparing a polyester from ketene and crotonaldehyde, decomposing the polyester with hydrochloric acid having a concentration of 35% by weight or more at temperatures ranging from room temperature to around the boiling point of the hydrochloric acid used, cooling the reaction mixture, separating a crude sorbic acid by filtration, washing the crude sorbic acid with water, putting the washed crude sorbic acid into water, heating and dissolving the mixture to yield a solution, adding activated carbon to the solution, boiling the mixture, and filtering the mixture while heating, and gradually cooling the resulting filtrate to yield a crystalline sorbic acid.
Japanese Examined Patent Application Publication No. 6-27097 discloses a process for preventing the deterioration of sorbic acid. The process includes the steps of retaining a sorbic acid wet cake in an inert gas and holding the oxygen concentration in a system at 10 parts by volume or less. The sorbic acid wet cake is formed in a production process of sorbic acid and contains water and/or a solvent.
A problem of sorbic acid or an alkali sorbate purified according to the above techniques is a tendency to color after drying, which is induced by the formation of new impurities, to thereby deteriorate the commercial value of a resulting product, although no deterioration of appearance as a solid mater or an aqueous solution is observed before drying. In this connection, even if a wet cake of sorbic acid is stored in an inert gas as in the above publication, the degree of coloring of sorbic acid increases with the passage of time after drying, when a color-inducing substance is formed before the storage. Desired advantages according to this technique cannot be significantly obtained.
Accordingly, an object of the invention is to provide a process for producing sorbic acid or its salt, which can prevent the formation of a new color-inducing substance in a purification operation of sorbic acid or its salt, where the obtained sorbic acid or its salt has a minimized degree of coloring and a minimized deterioration of hue over time.
The present inventors made intensive investigations to achieve the above object, and found that when a slurry or solution of sorbic acid or its salt in production operations of sorbic acid or its salt is left standing, the hue of a sorbic acid or its salt obtained by separating the slurry or solution is deteriorated with time, and that this deterioration of hue with time can be prevented by holding the slurry or solution of sorbic acid or its salt in an atmosphere at an oxygen concentration under a specific level. The present invention has been accomplished based on these findings.
Specifically, the invention provides a process for producing sorbic acid or its salt. The process includes the step of retaining a slurry or solution containing sorbic acid or its salt while holding an oxygen concentration of a gaseous phase at 4% by volume or less, which gaseous phase is in contact with the slurry or solution containing sorbic acid or its salt.
In the above production process, the oxygen concentration of the gaseous phase may be held at 4% by volume or less while introducing an inert gas into a gaseous phase of a reservoir holding the slurry or solution containing sorbic acid or its salt, or of a conduit adjacent to the reservoir, or of both. Such inert gases include, but are not limited to, a nitrogen gas. The oxygen concentration of the gaseous phase is preferably held at 1% by volume or less.
Slurries or solutions containing sorbic acid or its salt for use in the invented process are not critical and include slurries or solutions obtained by any process. Such slurries or solutions include, but are not limited to, slurries or solutions (particularly aqueous slurries or solutions) at each step in a purification operation of sorbic acid formed by the decomposition of a polyester, which polyester is obtained from ketene and crotonaldehyde, or in a purification operation of an alkali sorbate obtained by allowing an alkali to act upon the sorbic acid.
The reaction of ketene with crotonaldehyde is generally performed in the presence of a catalyst and in the absence of or in the presence of an inert solvent. Such catalysts include, for example, simple substances or compounds of manganese, cobalt, nickel, zinc, cadmium, and other transition metals; and pyridine, picoline, and other nitrogen-containing basic compounds. Examples of the compounds of the transition metals are oxides; salts of acetic acid, salts of isobutyric acid, salts of isovaleric acid, and salts of other organic acids; salts of sulfuric acid, salts of nitric acid, and salts of other inorganic acids; chlorides and other halides; acetylacetone complex salt, and other complex salts and complexes. Each of these catalysts can be used alone or in combination. The amount of the catalyst differs according to the type of the catalyst, but is generally about 0.1 to 10% by weight relative to the weight of ketene. The reaction of ketene with crotonaldehyde is performed at a temperature of, for example, about 2xc2x0 C. to 100xc2x0 C.
A reaction mixture containing a polyester obtained through the reaction of ketene with crotonaldehyde is usually distilled to remove unreacted crotonaldehyde and low boiling impurities, and is then subjected to a decomposition reaction. The polyester may be decomposed by any of hydrolysis with an acid or alkali or thermal decomposition, but should be preferably decomposed by hydrolysis with a mineral acid, especially with hydrochloric acid, for a high yield. The polyester is hydrolyzed at a reaction temperature of, for example, about 10xc2x0 C. to 110xc2x0 C. An extremely low reaction temperature will result in a deteriorated reaction efficiency, and in contrast, an extremely high reaction temperature may increase amounts of by-produced impurities such as tar substances. When the polyester is hydrolyzed with hydrochloric acid, the concentration of hydrochloric acid is, for example, about 15 to 40% by weight. An extremely low concentration of hydrochloric acid may invite a decreased reaction rate, and in contrast, an extremely high concentration of hydrochloric acid may invite disadvantages in handling property or operability. The amount of hydrochloric acid ranges, as hydrogen chloride, for example, from about 10 to 160 parts by weight relative to 100 parts by weight of the polyester.
A reaction mixture obtained through the decomposition of the polyester contains sorbic acid or its salt, a catalyst used in the reaction, as well as colored substances, tar substances, and other impurities by-produced in the reaction. Accordingly, the production of sorbic acid or its salt of high quality requires a purification operation. Such purification processes of a crude sorbic acid include, for example, (i) a process of preparing an aqueous solution of an alkali sorbate using a crude sorbic acid, treating the aqueous solution with activated carbon, acidifying the treated solution to yield a slurry of sorbic acid, and separating sorbic acid by filtration, (ii) a process of dissolving a crude sorbic acid in a hot water, treating the resulting solution with activated carbon, cooling the treated solution to yield a slurry of sorbic acid, and separating the slurry by filtration, (iii) a process of crystallizing sorbic acid with water or a mixture of water and an organic solvent (e.g., methanol, ethanol, or isopropyl alcohol) or further subjecting the crystallized sorbic acid to recrystallization, (iv) a process of dissolving a crude sorbic acid in petroleum, and distilling the resulting solution, and combinations of these processes. A purification process of a salt of sorbic acid includes, for example, a treatment with activated carbon.
The present invention has a feature in that an oxygen concentration of a gaseous phase in contact with a slurry or solution containing sorbic acid or its salt is held at 4% by volume or less where the slurry or solution is retained as intact for a predetermined period in such a production process of sorbic acid or its salt as mentioned above.
The present inventors found that when sorbic acid or its salt is present as a slurry or a solution in a production process thereof, the sorbic acid or its salt markedly deteriorates at a lower concentration of oxygen than sorbic acid or its salt as a wet cake. This is speculated to be induced by the following mechanism. In a slurry or solution containing sorbic acid or its salt prior to the completion of purification, impurities as precursors of colored substances are predominantly present in an aqueous phase, and the impurities are reacted with oxygen to readily deteriorate. These deteriorated impurities cannot be readily removed by rinsing crystals obtained by, for example, draining of a slurry, provably because of a high affinity of the impurities to sorbic acid or its salt, and are attached to and accompanied with crystals of sorbic acid or its salt to a drying operation and are converted into colored substances on exposure to heat. When an alkali salt of sorbic aid is prepared by neutralizing sorbic acid containing the deteriorated impurities with an alkali, the hue of an aqueous solution of the alkali sorbate after neutralization is deteriorated, and the color valency of an alkali sorbate obtained by concentrating and drying the aqueous solution is markedly deteriorated. In addition, when the alkali sorbate is stored in the air, the hue is further deteriorated with time.
However, according to the present invention, the oxygen of a gaseous phase is controlled to a markedly low concentration where sorbic acid or its salt is in a slurry or in a solution, and the resulting purified sorbic acid or its salt shows a markedly minimized deterioration of hue even in a long-term storage as a dried product in the air. This is provably because the deterioration of impurities in an aqueous phase is markedly suppressed and the impurities can be easily removed by, for example, rinsing with water and are not accompanied with crystallized sorbic acid or its salt. For example, if a sorbic acid slurry stored according to the invented process is filtered to yield a wet cake of sorbic acid, and the wet cake is neutralized to yield an alkali sorbate, the hue of the resulting product is of equal quality to an alkali sorbate obtained by filtering the sorbic acid slurry immediately after preparation to yield a sorbic acid wet cake and neutralizing the wet cake. Such hue of products includes the hue of the alkali sorbate aqueous solution, the hue of an alkali sorbate obtained by concentrating and drying the alkali sorbate aqueous solution, and the hue of the alkali sorbate after a long-term storage in the air.
According to the invented process, it is not necessary to feed a slurry or solution containing sorbic acid or its salt to a subsequent operation immediately after preparation, and a required portion of the slurry or solution can be fed to the subsequent operation at desired point of time. Such subsequent operations include, for example, a filtration operation, a treating operation with activated carbon, and an acidifying operation. A plan for the production of sorbic acid or its salt can be easily set up to provide an improved production efficiency.
The oxygen concentration of a gaseous phase in contact with the slurry or solution containing sorbic acid or its salt may be held at 4% by volume or less by simple replacement of the air in a gaseous phase of a reservoir containing the slurry or solution or of a conduit adjacent to the reservoir by an inert gas. Especially in the use of a slurry, the slurry is usually homogenized by agitation to facilitate supply of the slurry, and a contact surface with the air is continuously renewed and a gaseous phase is readily caught in a liquid phase. The oxygen concentration is therefore effectively held under a specific level while continuously or intermittently introducing an inert gas to the gaseous phase of the reservoir and/or the conduit.
The gaseous phase of the slurry or solution of sorbic acid or its salt has an oxygen concentration of preferably 1% by volume or less, more preferably 0.5% by volume or less, and particularly preferably 0% by volume. In this case, the slurry or solution is completely sealed with an inert gas. Generally, the product sorbic acid or its salt is more deteriorated with an increasing temperature. However, if the slurry or solution is completely sealed with an inert gas, the product sorbic acid or its salt has a significantly stable color value even if the slurry or solution rises in temperature to about 50xc2x0 C.
Inert gases for use in the invention include, for example, nitrogen gas, carbon dioxide gas, and argon gas, and nitrogen gas is commercially advantageously used.
As described above, a slurry of sorbic acid stored according to the invented process can yield a wet cake of sorbic acid having a higher quality by draining the slurry. Impurities which are not deteriorated and are holding water-solubility can be easily removed by rinsing the wet cake with water. In the draining and rinsing procedures, the oxygen concentration in a system should be preferably minimized with an inert gas, especially to 0%. Sorbic acid can be obtained by drying the wet cake of sorbic acid, or crystallizing or recrystallizing the same according to necessity.
Alkalis for use in the neutralization of the wet cake to yield an alkali sorbate include, for example, hydroxides, carbonates, and bicarbonates of sodium, potassium, and other alkali metals. Of these alkalis, potassium hydroxide or potassium salts should be advantageously employed for obtaining an alkali sorbate with a less deterioration over time. An alkali sorbate aqueous solution after the completion of neutralization is concentrated, is dried, and is granulated according to a conventional known technique.
The product sorbic acid or its salt can be used as preservatives for foods such as fish pastes, butters, cheeses, bean pastes, and jams.
The invented process, where the oxygen concentration of a gaseous phase is held under a specific level, can prevent the formation of new color-inducing substances in a purification operation of sorbic acid or its salt and can easily produce a high quality sorbic acid or its salt having a highly stable hue over time.